added Makefile check

Makefile

@@ -9,6 +9,9 @@ SOURCEDIR     = .
 BUILDDIR      = ./docs/html
 CURRENT_DIR   = $(shell pwd)
 
+check_python:
+	@python3 -c 'import sys; assert sys.version_info >= (3, 9), "Python 3.9+ is required"'
+
 # Put it first so that "make" without argument is like "make help".
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
@@ -18,6 +21,7 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
+	@$(MAKE) check_python
 	git submodule init
 	git submodule update --init --force --remote
 	if [ ! -d $(BUILDDIR) ]; then mkdir -p $(BUILDDIR); fi